1. Field of the Invention
The present invention relates to an electric power steering apparatus which employs an electric motor to provide power to assist the steering force for a vehicle.
2. Description of Related Art
The electric power steering apparatus serves to detect the steering torque and to assist the steering force by the rotation of an electric motor to be driven according to the detected torque.
FIG. 1 is a block diagram showing an outline construction of the electric power steering apparatus. The output, of a torque sensor 2 provided on a steering shaft (not shown) is inputted through an interface 3 into a CPU 1. In CPU 1 first an A/D conversion 11 is carried out and then a phase compensation 12 performed. The output of a vehicle speed sensor 7 is also inputted into CPU 1. In a memory of the CPU 1, there is stored a table 13 of the relations between the detected torques and vehicle speeds and the target current values of the drive current for an electric motor 5, and the target current corresponding to the inputted torque and vehicle speed is read out. Also, in CPU 1 the output of A/D conversion is differentiated 18, and this result is added to the read-out target current 14. The addition of the differentiated value is for the purpose of the inertial compensation of the motor 5.
In the CPU 1, the deviation is calculated 15 between the added result and the feedback drive current of the motor 5 which is detected by a drive current detector 6 and the A/D conversion 19 is carried out. The deviation is subjected to a PI calculation 16 and a control variable is calculated, and a PWM wave signal 17 corresponding to the variable and the signal indicating the rotation direction of the motor 5 is produced. The PWM signals are applied to a driving circuit 4 which is constructed as an H-type bridge of four switch transistors for a motor 5.
FIG. 2 is a flow chart showing the operating steps of the conventional PI calculation 16 in the CPU 1. The difference .DELTA. I.sub.n is obtained between the target current I.sub.M as the added result 14 and the detected driving current I.sub.s of the motor 5 (S1). Next, the integral element I.sub.n (expressed as a voltage) is calculated (S2) as follows; EQU I.sub.n =.DELTA.I.sub.n .times.K.sub.I (K.sub.I :constant)
Next, the difference .DELTA. P.sub.n is calculated between .DELTA.I.sub.n and the former difference .DELTA.I.sub.n-1 (S3) as follows; EQU .DELTA.P.sub.n =.DELTA.I.sub.n -.DELTA.I.sub.n-1
then, the proportional element P.sub.n (expressed as a voltage) is calculated as follows (S4); EQU P.sub.n =.DELTA.P.sub.n .times.K.sub.p (K.sub.p :constant)
And, the target voltage value V.sub.M to be given to the motor 5 is calculated (S5) as follows; EQU V.sub.M ={(V.sub.O +I.sub.n +P.sub.n)+V.sub.O }/2
then, the data are renewed (S6). After this, the duty cycle of the PWM wave required to realize the target voltage is calculated.
The motor 5 is an inductive load type, so that the current does not need to flow when the PWM wave duty cycle is small. For reasons such as the capacity of the CPU 1, the PWM wave has a frequency of about 20 kHz. It this frequency, the current can flow in the motor more than 50% of the PWM duty cycle. Then, the constants K.sub.I, K.sub.P are set up so that the stable control may be performed at more than 50% of the PWM duty cycle. As a result, there was a problem that the response was too late until the duty cycle reached over 50% at the rise of the motor current.
This is because I.sub.M and I.sub.S are all small, .DELTA.I.sub.n is small, and it is required that the operating steps of S1 to S6 are repeatedly carried out over a considerable number of times until V.sub.M or the duty cycle becomes a certain large value. Owing to this poor response, there was a problem that when the steering was repeated right and left, the assistance force was behind the steering force, and the operator felt vibration.